Mammography is the process of obtaining x-ray images of the human breast for diagnosis or surgery. It involves positioning a patient's breast on a breast support platform of a mammography unit and exposing the breast to x-rays to create a latent image of the breast on an image receptor. The image receptor is a film, in contact with an intensifying screen that fluoresces when struck by x-rays. The film and intensifying screen are contained in a cassette which is located in a cassette holder directly beneath the breast support platform. After exposure to x-rays, the film is removed from the cassette and developed to produce a radiographic image of the breast.
As is known to those with skill in the art, before the breast is exposed to x-rays, it is compressed against the breast support platform by a mammography compression device. This is done in order to reduce patient radiation dose, patient motion, and scattered radiation to the film, and to increase image contrast and improve image quality.
The mammography compression device consists of a rectangular flat plate, called a compression paddle or a compression plate, that is attached to the mammography unit. The edges of the plate are turned upward away from the breast support platform to provide a smooth edge for the patient. The compression paddle is located between the breast support platform and the x-ray tube of the mammography unit. The plane of the compression paddle is parallel to the breast support platform and perpendicular to the axis of the x-ray beam. The compression paddle is movable along a line parallel to the axis of the x-ray beam. The compression paddle is usually made of thin, plastic that absorbs only a small fraction of the incident x-ray beam.
For clinical use of the mammography compression device, the breast is positioned on the breast support platform in the desired orientation. This is facilitated by a light beam originating from the x-ray tube assembly that passes through the compression paddle and illuminates the area of the breast support platform that will be exposed to x-rays.
The compression paddle is moved either manually or by power drive to apply a compression force to the breast, thus flattening the breast against the breast support platform to a near uniform thickness. An x-ray exposure is then made. After the x-ray exposure the compression force is released and the breast recovers its normal shape.
A complete mammographic study usually involves at least two x-ray exposures of each breast, 1) a cranio-caudad view in which the breast is compressed in a superior-inferior direction, i.e. from the direction of the patient's head downward against the breast lying on the breast support platform. The plane of the breast support platform is parallel to the floor and the x-ray beam is directed vertically downward; and 2) a lateral or oblique view in which the breast is compressed medio-laterally, i.e., from midline sidewise against the breast support platform which is angled, along with the axis of the x-ray ray beam, relative to the floor.
Adequate compression is an important factor in achieving good quality mammograms, and mammography facilities that are accredited by the American College of Radiology (ACR) and certified by the U.S. Food and Drug Administration under the Mammography Quality Standards Act of 1992 (MQSA) are required to test their mammography compression devices periodically. Although there are a number of test tools that can be used (1), the method recommended by the ACR (2), and usually employed, consists of positioning a bathroom type scale on the cassette holder, and applying a compression force to the scale either manually or remotely using the compression paddle attached to the x-ray unit. The final scale reading is recorded manually in a log. The test records are subject to periodic review by authorized MQSA personnel. The breast support platform and compression paddle are protected by towels or foam pads before pressure is applied to the scale.
Other known test tools for measuring the force exerted by mammography compression devices include hydraulic load cells, industrial strain gauges, compression force gauges made of compressible polyethylene packing material, and extension type scales.
Although the bathroom type scale is the most widely used method for testing mammography compression devices there is no standard scale designed expressly for this purpose, and in practice the scales vary considerably in size, shape, weight, sensitivity and accuracy. The aforementioned other prior methods for testing mammography compression devices do not satisfy long-felt needs in the art for methods and apparatus to measure the magnitude of compression force applied by mammography compression devices that are quick, efficient, and standardized to particular types of mammography units.
It is also known in the field of radiology to utilize inanimate objects is place of patients in conducting serial x-ray exposures for calibration, image quality, and dosimetry purposes. Such an object is called a phantom and in mammography the object is called a breast phantom. The inventor of the subject matter herein claimed and disclosed has recognized a long-felt need in the art for an inanimate object designed specifically for testing mammography compression devices. The inventor calls such a device a compression phantom.
It is also well known in the field that bathroom type scales presently used to test mammography compression devices are usually heavy and need to be supported by a chair or other support in order to perform the test when the x-ray beam is angled off the vertical. The inventor of the subject matter herein claimed and disclosed has also recognized additional long-felt needs in the art for methods and apparatus that are light weight and capable of measuring compression force, not only for vertical downward projections of the x-ray beam, but also for lateral and oblique projections.
The inventor of the subject matter herein claimed and disclosed has recognized a need in the art for a compression phantom that can be used with or incorporated into a breast phantom for testing mammography image quality.
The inventor of the subject matter herein claimed and disclosed has recognized further long-felt needs in the art for methods and apparatus to measure the uniformity of compression force across the plane of the compression paddle. Methods and apparatus to generate permanent direct records of compression force tests for quality assurance and medico-legal needs are also desired in the art.